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        let num1 = 123,num2 ='hello';
        console.log(num1);
        console.log(num2);
        console.log(typeof(num1)); //打印数据类型
        console.log(typeof(num2));
        console.log('-----------');
        let flag = true;
        console.log(flag);
        console.log(typeof(flag));
        flag = false;
        console.log(flag);
        console.log(typeof(flag));
        console.log('-----------');
        let str1 = '子曰："学而时习之"';
        console.log(str1);
        console.log(typeof(str1));
        //子曰：‘学而时习之’
        //he said:'day day study';
        //在字符型中，单引号中用单引号，或双引号中用双引号，需要用到转义符'/'
        let str2 = 'he said:\'day day study\'';
        console.log(str2);
        console.log(typeof(str2));
        str2 = "子曰：\"学而时习之\"";
        console.log(str2);
        console.log('=----------------');
        let Null = null;
        console.log(Null);
        console.log(typeof(Null));
        console.log('------------');
        let unNum;
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        console.log(typeof(unNum));


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